The present invention relates generally to resolution enhancement technology (RET) and more particularly to methods and systems for enhancing resolution of compressed image data.
Resolution Enhancement Technology (RET) is a well-established technology for the improvement of printed text. Resolution enhancement of input image data involves pattern-matching techniques to smooth text edges and avoid jaggedness. A large number of methods exist. In general, with RET techniques the edges of letters are recorded and diagonal edges are smoothed. A number of these RET techniques are summarized below.
U.S. Pat. No. 5,282,057 to Mailloux, et al. discloses a bit-map image resolution converter for converting binary image data originally generated at a lower resolution into representative binary image data of a higher resolution, wherein the conversion ratio, or magnification factor, is an integer value. This patent also discloses methods for smoothing the interpolated output image to thereby reduce objectionable visual characteristic that are observable in digitally encoded data using conventional magnification techniques.
U.S. Pat. No. 5,387,985 to Loce, et al. teaches a non-integer image resolution conversion using statistically generated look-up tables for converting the resolution of bitmap images, and using a template matching process to alter the resolution of digital images for printing or similar methods of rendition. Statistically generated templates, implemented using look-up tables, are used to improve document appearance by converting from an original input spatial resolution to an output spatial resolution that is device dependent, where there is a non-integer relationship between the input and output resolutions. The resulting image signals may then be utilized to control a scanning beam where the beam varies in intensity and duration according to the pulses used to control it.
U.S. Pat. No. 5,579,445 to Loce, et al. discloses a method and apparatus for automating the design of morphological or template-based filters for print quality enhancement. A plurality of different phase, but same resolution, subsampled images are generated from training documents. Statistical data derived therefrom is then employed in an automated process to generate filters. The filters may be used for resolution enhancement and/or conversion of bitmap images. Furthermore, the statistical data is used to produce filters that are intended to not only optimize image structure, but image density as well.
U.S. Pat. No. 5,696,845 to Loce, et al. teaches a method and apparatus for improving the appearance of printed documents, and more specifically, using a template matching process to enhance the fast-scan resolution of digital images while maintaining raster resolution for printing. Multiple-bit per pixel, statistically generated templates, implemented using look-up tables, are used to improve document appearance by converting from a single-bit per pixel to N-bits per pixel, while preserving raster resolution of the printed output. The resulting N-bit per pixel image signals may be utilized to control a scanning beam where the beam varies in intensity and duration according to the pulses used to control it.
U.S. Pat. No. 5,724,455 to Eschbach discloses an automated template design method for print enhancement for automating the design of pattern matching templates used to implement a print quality enhancement filter. More particularly, the method is directed to the use of a representative set of page description language decomposed training documents, and statistical data derived therefrom, in an automated process which generates templates for resolution enhancement and/or conversion of bitmap images.
The above described RET techniques use template-matching filters and are applied basically to bitmap pixel data not to compressed data. In resolution enhancement of input image data, the pixels are not only magnified by replication in each direction but a template filter is also applied to smooth edges and correct jaggedness. FIG. I is a simple block diagram in which the replication and template matching filtering are applied to the compressed data. If the image containing text is compressed with CCITT compression schemes, the enhancement involves the steps of decompressing the input image data, replicating the compressed data, applying template-matching filter to the replicated data, and recompressing the resolution enhanced pixel data.
As shown in FIG. 1, the compressed data is input and decompressed in the decompressing unit 101. The decompressed data is applied to the replication unit 102 and the number of pixels is magnified in replication unit 102 to enhance the resolution of the input image. The replicated pixel data is sent to the template matching filter 103 and the edge of replicated pixel data is smoothed in the template matching filter 103. The replicated and smoothed pixel data is compressed again in recompressing unit 104. From this figure, a decompressing unit 301 and a recompressing unit 304 are needed to apply the template-matching filter to compressed data.
FIGS. 2A-2I depict the bitmap examples of each output in FIG. 1 and each group of FIGS. 2A-2C, 2D-2F and 2G-2I illustrates an example of up-scaling as 2xc3x972, 3xc3x973 and 2xc3x971, respectively. FIG. 2A is the output of decompressing unit 101, which is decompressed bitmap pixel data. FIG. 2B is the output of replication unit 102 in which the bitmap of decompressed pixel data in FIG. 2A is replicated by 2xc3x972. FIG. 2C is the output of template matching filter 103 in which the bitmap of pixel data up-scaled by 2xc3x972 in FIG. 2B is edge smoothed. FIG. 2D is the output of decompressing unit 101, which is decompressed bitmap pixel data. FIG. 2E is the output of replication unit 102 in which the bitmap of decompressed pixel data in FIG. 2D is replicated by 3xc3x973. FIG. 2F is the output of template matching filter 103 in which the bitmap of pixel data up-scaled by 3xc3x973 in FIG. 2E is edge smoothed. FIG. 2G is the output of decompressing unit 101, which is decompressed bitmap pixel data. FIG. 2H is the output of replication unit 102 in which the bitmap of decompressed pixel data in FIG. 2G is replicated by 2xc3x971. FIG. 2I is the output of template matching filter 103 in which the bitmap of pixel data up-scaled by 2xc3x971 in FIG. 2H is edge smoothed.
In order to obtain the resolution enhanced pixel data, the device in FIG. 1 needs amount of buffering and processing steps, which may cost up and be inappropriate in low-end level application.
The present invention provides methods and systems for increasing the resolution of image data. With the present invention all resolution enhancing technology (RET) operations may be performed in the compressed domain, so that the resolution enhancement is applied to the transition maps and not to pixel image data. The present invention provides a simple manipulation of the compressed image data, which effectively increases the image of resolution and smooth edges in the image, while avoiding full decompression and recompression of the data. This results in significant savings in time and computational overhead.
In accordance with one aspect of the present invention, a method is practiced in a compressed domain to enhance a resolution of input image data. For the purpose of the present invention, (a) the transitions of the input lines of the compressed image data are analyzed. (b) The positional differences between the closest compatible transitions in the input lines are calculated. (c) Distances of transitions from a reference position in each input line are magnified. (d) A line is interpolated between the input lines and the transitions of the interpolated line are determined according to the positional differences. (e) The corrected transitions of each line are encoded to compressed codes.
In accordance with another aspect of the present invention, a method is practiced in a compressed domain to enhance a resolution of input image data. For the purpose of the present invention, (a) the transitions of the input lines of the compressed image data are analyzed. (b) Distances of transitions from a reference position in each input line are magnified. (c) The positional differences between closest compatible transitions in the magnified transitions of input lines are calculated. (d) The positional differences are compared with a limit number. (e) A line is interpolated between the input lines and transitions of the interpolated line are determined according to the positional differences and the limit number. (f) The corrected transitions of each line are encoded to compressed codes.
In accordance with another aspect of the present invention, an apparatus operates with data in a compressed domain to enhance a resolution of input image data. For the purpose of the present invention, a decoder maps the compressed image data to the transitions of the input line. A processor magnifies the distances of transitions from a reference position in each input line. The processor also interpolates a line between the input lines. The transitions of the interpolated line are determined according to the transitions of input line. An encoder encodes the corrected transitions of each line into compressed codes.
In accordance with another aspect of the present invention, a computer program for controlling a computer to enhance a resolution of compressed image data is stored in a recording medium readable by the computer. A means is recorded on the recording medium for analyzing transitions of compressed image data in input lines. A means is recorded on the recording medium for calculating positional differences between the closest compatible transitions of input lines. A means is recorded on the recording medium for magnifying distances of transitions from a reference position in each input line. A means is recorded on the recording medium for interpolating a line between the input lines where transitions of the interpolated line are determined according to the positional differences. A means is recorded on the recording means for encoding the corrected transitions to compressed codes.
In accordance with another aspect of the present invention, the present invention is applied to a facsimile machine for enhancing a resolution of compressed image data. The facsimile machine includes: an analyzer for analyzing transitions of compressed image data in input lines; a calculator for calculating positional differences between the closest compatible transitions of input lines; a magnifier for magnifying distances of transitions from a reference position in each input line; an interpolator for interpolating a line between the input lines, where transitions of the interpolated line is determined according to the positional differences; and an encoder for encoding the corrected transitions to compressed codes.
The present invention provides a system where a simple RET operation is performed to increase the resolution by an integer factor in each direction. Performing RET in the compressed domain gives potential speed and memory improvements over spatial domain processing. The effects of the invention is to replicate pixels horizontally and duplicate lines by interpolation. The number of the transitions in present invention is commonly much smaller than the number of pixels in a row. The process provided in the present invention achieves savings in memory and computation by not computing pattern-matching filter.
The RET performed in the compressed domain can be achieved at potentially lower processing and memory requirement. It is not necessary to apply template matching to every pixel in the image, thus saving operation and reducing complexity. It is also avoid the process of converting the transitions to rasters and the more expensive process to convert from rasters to transitions (run length or offset counting).